Single and superimposed bedforms: a synthesis of San Francisco Bay and flume observations

Sedimentary Geology



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Tidal currents with maximum depth-averaged velocities ranging up to 250 cm/sec have generated ripples, two- and three-dimensional sand waves, and upper flat beds on the floor of central San Francisco Bay. Determination of the hydraulic conditions under which the observed beds exist, indicates that the bed configuration at any point in the bay is a function of the local velocity, sediment size, and depth. The bay observations, for flows up to 85 m deep, were combined with shallow-flow observations and a single set of bed-phase boundaries was determined for the combined data. Critical shear velocities calculated for the transitions from ripples to sand waves and from sand waves to upper flat beds, in flows tens of meters deep, are within 10% of critical shear velocities observed for the same transitions in flume flows only tens of centimeters deep. Comparison of bedform sequences suggests that, for flows up to tens of meters deep, beds of 0.25-0.50 mm sand respond to increasing flow velocities by forming ripples, two-dimensional sand waves, three-dimensional sand waves, and flat beds. At any constant depth, equilibrium sand waves increase in height and migration rate as flow velocity increases. The wavelength and maximum height of both two- and three-dimensional sand waves increase with depth also, but migration rates decrease. Because the maximum size of both kinds of bedforms varies with depth, classification schemes based on size arbitrarily separate genetically similar bedforms. In the bay, in contrast to flumes, sand waves having the largest height-to-depth ratios occur in relatively coarse sand. Tidal and seasonal velocity fluctuations are interpreted to be more destructive to finer-grained sand waves, because in finer grain sizes sand waves are stable at a relatively narrow range of velocities. Ripples, sand waves, and upper and lower flat beds are commonly superimposed on larger bedforms. Small bedforms can exist in equilibrium on the larger bedforms because the large bedforms generate boundary layers in which the small bedforms are locally stable. The distribution of small bedforms superimposed on larger bedforms reflects lateral and vertical variations in shear velocity in flow over large bedforms. ?? 1980.

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Single and superimposed bedforms: a synthesis of San Francisco Bay and flume observations
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Sedimentary Geology
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Sedimentary Geology
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